Methods and systems for synchronized ultrasonic real time location

ABSTRACT

Methods and systems for determining a location and an identity of a portable device are provided. The system includes apparatus for transmitting timing synchronization information, a plurality of stationary ultrasonic base stations and a plurality of portable devices. Each ultrasonic base station is configured to receive the timing synchronization information and to transmit a corresponding ultrasonic location code in a time period based on the received timing synchronization information. Each portable device is configured to: 1) receive the timing synchronization information, 2) detect the ultrasonic location codes from the ultrasonic base stations and 3) transmit an output signal including a portable device ID representative of the portable device and the detected ultrasonic location code. Each portable device is synchronized to detect the ultrasonic location code in the time period based on the received timing synchronization information.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of priority from U.S. patentapplication Ser. No. 12/016,547, filed Jan. 18, 2008 and U.S.Provisional patent application No. 60/881,269, filed Jan. 20, 2007. Thecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates real-time location systems. In particular,the present invention relates to methods and systems for locating andidentifying portable devices in an enclosure.

BACKGROUND OF THE INVENTION

Real-time location (RTL) systems have become popular in recent years.Conventional RTL systems typically use radio frequency (RF) transmissionto determine location. The RF based methods typically do not provideenough accuracy to support room and sub-room level accuracies. A fewsystems utilize infrared (IR) transmission as a localization method. Theconventional “IR method” typically employs an IR transmitter on aportable device (i.e. a tag) and IR receivers in base stations that arescattered in rooms and corridors within the enclosure. A portable deviceID is typically received by one of IR the base stations and the locationof the portable device is determined based on its vicinity to a basestation. A tag based IR transmitter typically needs a line of sightbetween the transmitter and a receiver (i.e. a base station) in order torobustly detect the device ID. This type of RTL system, thus, isoccasionally prone to a lack of reception by the receiver base stationswhen the line of sight is blocked, making it susceptible to reliabilityproblems. Also, these types of IR base stations are presently wired bothfor power and connectivity because they use IR receivers that are “open”at all times. The need for wiring increases the installation complexityand cost.

There is also another type of IR system that employs IR transmitters atthe base stations and IR receivers at the portable device which attemptsto solve the problem of poor IR sensitivity, by transmittingsubstantially higher power levels than wired stationary base stationsare typically capable of. This system may address the sensitivityproblem but does not address the wired installation problem.Furthermore, in order to make sure that the IR signal is readilyavailable to the portable device at all times, the IR base stationstransmit the IR signals at a very high burst repetition rate. The firstinstallations of such systems generally failed because of an unexpectedproblem; the IR base stations interfered with TV remote controls thatare a part of almost all patient rooms in hospitals. To solve theproblem, newer systems transmit the IR signals much less often, with asevere penalty on tag power consumption (because the tag needs to searchfor the IR signal with an open IR receiver).

SUMMARY OF THE INVENTION

The present invention relates to systems and methods for locating andidentifying portable devices in an enclosure. The system includes meansfor transmitting timing synchronization information, a plurality ofstationary infrared (IR) base stations and a plurality of portabledevices. Each IR base station is configured to receive the timingsynchronization information and to transmit a corresponding IR locationcode in a time period based on the received timing synchronizationinformation. Each portable device is configured to 1) receive the timingsynchronization information, 2) detect the IR location codes from the IRbase stations and 3) transmit an output signal including a portabledevice ID representative of the portable device and the detected IRlocation code. Each portable device is synchronized to detect the IRlocation code in the time period based on the received timingsynchronization information.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 is a block diagram of a system for locating and identifyingportable devices in an enclosure, according to an embodiment of thepresent invention;

FIG. 2 is a block diagram of a radio frequency (RF) base station,according to an embodiment of the present invention;

FIG. 3 is a block diagram of an infrared (IR) base station, according toan embodiment of the present invention;

FIG. 4 is a block diagram of a portable device, according to anembodiment of the present invention;

FIG. 5 is a block diagram of a system for locating and identifyingportable devices using multiple subnets, according to another embodimentof the present invention;

FIGS. 6A and 6B are graphs of various transmissions in synchronizationand communication layers of the system shown in FIG. 5 as a function oftime, according to an embodiment of the present invention;

FIG. 7 is a flow chart illustrating a method for determining thelocation and identity of a portable device, according to an embodimentof the present invention;

FIGS. 8A and 8B are diagrams of transmissions versus time of an IR basestation and a portable device, according to an embodiment of the presentinvention;

FIG. 9 is a block diagram illustrating an ordering of IR base stationsin an enclosure such that the IR base stations transmit at differenttime periods, according to another embodiment of the present invention;and

FIGS. 10A, 10B and 10C are diagrams of transmissions versus time ofordered IR base stations and a portable device, according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

RTL systems are known that include base stations that transmit IRsignals (i.e., IR base stations) with their respective base station ID(BS-ID) to portable devices that are equipped with IR receivers. Uponreception of a BS-ID, the portable device transmits both the device IDand the received BS-ID to a reception device, for example, by radiofrequency (RF) or IR transmission. The RTL system may include a serverthat associates the BS-ID with the device ID (received from the portabledevice by the reception device). In this manner, the position of theportable device may be displayed as the position of the IR base station.In typical implementations, base stations are connected to a powersupply whereas the portable device is typically is powered by a coincell battery, a much higher transmitted power may be achieved by using abase station for IR transmission, as compared to portable device (tag)for IR transmission. As a result the reliability of the reception isimproved.

The IR base station typically transmits a BS-ID (as well as otherinformation) continually or almost continually and, thus, may notoperate via battery power. Accordingly, the portable device has noindication of when the transmissions by the IR base stations occur andmay receive the IR signals anytime (or almost anytime).

Because the base stations may not operate by battery power, typicallyone of two approaches is used for RTL systems with IR transmitter typebase stations. In the first approach, power may be routed from anexisting A/C outlet to a desired location for the IR base stationoperation. In the second approach, the base station may be connecteddirectly to an outlet.

The first approach may be costly as compared to the typical cost of abase station. In a typical installation, routing power (or power overEthernet) can amount to costs of greater than five times the cost of thebase station itself. In the second method, the routing installationcosts may be mitigated, but, the direct connection to the outlettypically compromises the system performance. For example, most poweroutlets are installed at the bottom of walls. In the position near theoutlets, base stations may be susceptible to being blocked by objectsand they typically cover twice a range that a ceiling mounted basestation covers (for example, if the base station is installed in themiddle of a room).

In addition, IR base stations typically do not coexist in a samephysical open space because “dead zones” may be generated where the IRsignals from the two base stations overlap and because the IRtransmissions from the base stations are typically unsynchronized. Ifthe base station uses a short random duty cycle for transmission, theportable devices may operate in a receiving state for a much longerperiod, thus resulting in a substantially reduced battery lifetime.Synchronizing the portable device to the transmission rate of the IRbase station is generally not desirable. For example, in many cases, theportable device is outside of the range of an IR base station (such asin corridors) that is to say, in an area typically not covered by theRTL system. In such cases, the portable device typically activates theIR receiver for longer periods of time, waiting for a possible IRtransmission from the IR base station.

The present invention relates to systems and methods for detecting andlocating portable devices in an enclosure. According to one aspect ofthe present invention, the exemplary system includes a RF base stationthat may periodically transmit a beacon. The beacon may be received byboth the IR base stations and the portable devices (i.e. nodes of thesystem). In an exemplary embodiment, the beacon includes timingsynchronization information (TSI) that may provide a unified time oforigin to all nodes in the system. After a predefined period of timefrom the unified time of origin, the IR base stations may transmit theircorresponding BS-IDs. The portable device may open communication toreceive the BS-ID after the same predetermined period of time from theunified time of origin. The portable device may then transmit itsassociated ID and the received BS-ID to the RF base station. Bysynchronizing the IR base stations and the portable devices to a unifiedtime of origin, the portable device may be in a sleep mode when it isnot in a state of transmission or reception. Accordingly, a batterypower consumption by the portable device may be reduced.

According to another aspect of the present invention, the exemplarysystem may include a plurality of RF base stations (also acting as nodesin the system), where the RF base stations act as synchronizing basestations other RF base stations. Each RF base station may periodicallytransmit a beacon to associated IR base stations, other RF base stationsand any associated portable devices. The transmission from the portabledevice may be received by the synchronizing base stations or by theother RF base stations. The RF base stations may provide the device IDand the associated BS-ID to a central computer. The information may thenbe displayed or processed by other client applications.

Referring now to FIG. 1, a block diagram is shown of a system 100 fordetermining a location and an identity of portable devices 108 in anenclosure 102. System 100 may include a plurality of IR base stations(IR-BS's) 106, one or more portable devices 108 and at least one RF basestation (RF-BS) 110.

RF base station 110 may transmit timing synchronization information(TSI), for example, in a beacon, to IR base stations 106 and portabledevices 108 that are each equipped with RF transceivers, by RFtransmission. The TSI may be used by IR base stations 106 to transmit acorresponding BS-ID, i.e. an IR location code, in a period of time afterreceiving the TSI. The TSI may be used by portable devices 108 to open areception channel in the period of time corresponding to thetransmission time of the IR base stations 106. The period of time for IRbase stations 106 to transmit the respective BS-ID signals may be fixedor transmitted as a part of the information carried by the beacon.

Enclosure 102 may include a plurality of separate zones 104, whichtypically coincide with individual rooms or zones within enclosure 102.For example, zone 104-1 represents a corridor. Each room or zone may beprovided with at least one IR base station 106. For example, corridor104-1 includes IR base stations 106-1, 106-2, 106-3.

The IR base stations 106 may be mounted from a ceiling or on walls ofrooms 104 or mounted at an AC receptacle (not shown). The IR basestations 106 may be battery powered or may be activated from anavailable power source, described further below with respect to FIG. 3.Even if an IR base station 106 is positioned at a lower room level, suchas near an AC receptacle, and an obstruction is located nearby, IRcommunication may still be possible due to IR reflections from a ceilingor a wall of the room 104.

IR base station 106 typically transmits very short bursts of IR locationsignals from an IR transmitter (i.e. corresponding BS-IDs) at periodicintervals based on the TSI received from RF-BS 110. Each IR base station106 may transmit a unique BS-ID signal that may be identified at acentral control as originating from a particular zone or room 104. TheBS-ID may, for example, be transmitted at an IR carrier frequency thatis typically around 40 kHz that is resilient to avoid signalinterference from fluorescent lamps and may be in the form of bursts ofthe order of about 3 milliseconds long. It is understood that anysuitable frequency and duration of the IR burst may be used. Although IRbase station 106 is described as including an IR transmitter, it iscontemplated that IR base station 106 may also include an IR receiver.

Portable devices 108 may be provided for persons or apparatuses. Theportable devices 108 may include an IR receiver and an RF transceiver(FIG. 4) which are coupled to each other. In this manner, the RFtransceiver may receive the TSI and may transmit received BS-ID and itsdevice ID at an RF carrier frequency to RF base station 110.

The modulated carrier signal received by RF base station 110 may bedecoded to reproduce the BS-ID and the device ID, for example, by adecoding network (not shown), by server 112 or by RF base station 110.The BS-ID and the device ID may be used by server 112 to determine wherea particular person or device is located.

Although IR base stations 106 are described, it is contemplated that thebase stations 106 may also be configured to transmit a correspondingBS-ID by an ultrasonic signal, such that base stations 106 may representultrasonic base stations. Accordingly, portable devices 108 may beconfigured to include an ultrasonic receiver to receive the BS-ID froman ultrasonic base station.

Referring now to FIG. 2, a block diagram of RF base station 110 isshown. RF base station 110 may include local area network (LAN) chip202, microcontroller 204, RF transceiver 206, RF switch 208 and antennas210-1, 210-2. RF base station 110 may be powered by an external powersupply 212 or by an Ethernet power supply (POE) 214 that is also used topower LAN chip 202. LAN chip 202 may also be powered by external powersupply 212. LAN chip 202 may be used to allow multiple RF base stations110 to communicate with each other over a network such as Ethernet 216.For example, as described further below, multiple RF base stations 110may transmit a timing signal to each other via Ethernet 216 in order tosynchronize the RF base stations 110.

RF transceiver 206 may be configured to receive RF transmissions, forexample, from portable device 108 (FIG. 1) or from another RF basestation 110, from antenna 210. RF transceiver 206 may also be configuredto transmit the TSI, such as by transmitting an RF beacon that includesthe TSI, via antenna 210. In an exemplary embodiment, antennas 210-1,210-2 may be conventional diversity antennas used for both transmissionand reception alternately.

Microcontroller 204 may be configured to control LAN chip 202, and RFtransceiver 206, as well as RF switch 208, for example, to transmit theTSI, communicate with other RF base stations and receive RFtransmissions from portable devices 108 (FIG. 1). It is contemplatedthat RF base station 110 may be configured to receive RF transmissionsfrom IR base stations 106 that include a portable device ID and anassociated BS-ID. RF base station 110 may also be configured to connectto server 112 (FIG. 1) by Ethernet 216.

Referring now to FIG. 3, a block diagram of IR base station 106 isshown. IR base station includes RF transceiver 308, RF switch 310,antennas 312-1, 312-2, microcontroller 306, IR LED driver board 302 andIR transmitter 304. IR base station 106 may be powered by battery 314 orby an external power supply 316. Synchronized IR base stations 106 mayprovide for a simple and low cost installation and allow for acoexistence of IR base stations 106 without dead regions (typicallycaused by an overlap in coverage).

RF transceiver 308 may be configured to receive RF transmissions, forexample, beacons including the TSI from RF base station 110 (FIG. 1) viaantenna 312-1 or 312-2. Antenna 312-1 or antenna 312-2 may be selectedby RF switch 310 and that may be controlled by microcontroller 306. Forexample, microcontroller 306 may determine that antenna 312-2 receives astronger beacon signal as compared to antenna 312-1 and may control RFswitch 310 to select antenna 312-2 for RF reception.

Although not shown in FIG. 3, a BS-ID associated with IR base station106 may be stored by IR base station 106, for example, in a memory ofmicrocontroller 306. Driver board 302 may be configured to transmit theassociated BS-ID to IR transmitter 304. Although in an exemplaryembodiment, 64 LEDs are used as IR transmitter 304, it is understoodthat IR transmitter may include any device suitable for transmitting anIR burst that includes the associated BS-ID.

Microcontroller 204 may be configured to control driver board 302, andRF transceiver 308, as well as RF switch 310. Based on the TSI receivedby RF transceiver 308, microcontroller 204 may control driver board 302to transmit the BS-ID at a period of time, T1, after the TSI isreceived. For example, referring to FIG. 8A, a beacon 802 including theTSI may be received. After a period T1, IR transmitter 304 (FIG. 3) maybe controlled to transmit an IR signal 804 including the BS-ID. After aperiod T2, another beacon 802 may again be received by IR base station106 (FIG. 3). Although IR transmission 804 is illustrated as occurringonce every period T2, it is contemplated that an IR base stationtransmission 804 may be activated multiple times in time period T2.

Referring back to FIGS. 1 and 3, by synchronizing all of the nodes (i.e.the RF base station 110, IR base stations 106 and portable devices 108),IR base stations 106 may be powered by battery 314. Because portabledevices 108 are battery powered, if the nodes of system 100 were notsynchronized, the IR transmitter 304 would transmit IR signals moreoften (increasing its power consumption) so that portable device 108would reduce an amount of time detecting the IR signals (to decrease itspower consumption). IR detection may be power consuming and longdetection periods may quickly deplete the battery of the portable device108.

According to an exemplary embodiment, IR base station 106 may include atimer, for example, as part of microcontroller 306. The timer may beused by microcontroller 306 to begin IR transmission. IR base station106 may periodically adjust the timer by communicating a request to RFbase station 110 (FIG. 1) a time delay to a next beacon transmissiontime or the next IR transmission time.

IR base station 106 may transmit a request to RF base station 110(FIG. 1) via RF transceiver 308 for a time delay to a next beacontransmission time or the next IR transmission time. The RF base station110 (FIG. 1) may reply to IR base station 106 with an RF transmissionthat includes the time delay. IR base station 106 may then activate RFtransceiver 308 to receive the beacon at the received time delay and mayadjust the timer according to the received beacon.

In addition, RF transceiver 308 permits IR base station 106 to transmitan RF signal including information about the status of the transmitterof IR base station 106 to RF base station 110. For example, a batterystatus of IR base station 106 may be transmitted. If no RF transmissionsare received from IR base station 106, server 112 may determine that theparticular IR base station 106 is inoperable.

Referring now to FIG. 4, a block diagram of portable device 108 isshown. Portable device 108 includes IR detector 402, microcontroller404, motion detector 406, RF transceiver 408, RF switch 410 and antennas412-1, 412-2. Portable device 108 is powered by battery 414.Microcontroller 404 may control motion detector 406 to determine whetherportable device 108 is stationary or in transit. It is contemplated thatmotion of portable device 108 may be communicated to RF base station 110(FIG. 1), for example, to determine whether portable device 108 may bemoving out of the enclosure 102. More importantly, when motion detector406 is not active the portable device 108 may stay asleep as long asthere is no motion. The system 100 would know the location of portabledevice 108 the last time portable device 108 was located based on itslast transmission. Also, portable device 108 can transmit acommunication to the system that it is in a no-motion state and is aboutto go to sleep. In practice, sleeping portable devices 108 maycommunicate with the system once in about 5 minutes to let the systemknow that they are alive. In addition, IR base stations 106 may also beplaced in a sleep mode. For example IR base stations 106 in locationssuch as doctors' offices may be placed in sleep mode during hours wherethe office is closed.

IR detector 402 may be configured to receive IR transmissions, forexample, IR signals including associated BS-IDs, from IR base stations106 (FIG. 1). IR detector 402 may detect IR signals from a number of IRbase stations 106 (FIG. 1). Microcontroller 404 may be configured toselect a stronger one of the IR signals received from different IR basestations 106. Microcontroller 404 may store the BS-ID, for example in amemory of microcontroller 404 (not shown).

RF transceiver 408 may be configured to receive RF transmissions, forexample, beacons including the TSI from RF base station 110 (FIG. 1) viaantenna 412-1 or 412-2. Antenna 412-1 or antenna 412-2 may be selectedby RF switch 410 that may be controlled by microcontroller 404. Forexample, microcontroller 404 may determine that antenna 412-2 receives astronger beacon signal as compared to antenna 412-1 and may control RFswitch 410 to select antenna 412-2 for RF reception. RF transceiver 408may also be configured to transmit RF signals to RF base station 110(FIG. 1) via antenna 412-1 or 412-2, for example, RF signals includingthe device ID associated with portable device 108 and a BS-ID receivedfrom IR detector 402.

Microcontroller 404 may be configured to control IR detector 402, and RFtransceiver 408, as well as RF switch 410. Based on the TSI received byRF transceiver 408, microcontroller 404 may control IR detector 402 tobegin IR detection the BS-ID after a period of time, T1, that is afterthe TSI is received. For example, referring to FIG. 8B, a beacon 802including the TSI may be received by RF transceiver 408 (FIG. 4). Afterperiod T1, IR detector 402 (FIG. 4) may be controlled to begin receivingan IR signal 806 including the BS-ID. Microcontroller 404 may thencontrol RF transceiver 408 (FIG. 4) to transmit an RF signal 808including the associated device ID and the received BS-ID. After aperiod T2, another beacon 802 may again is be received by portabledevice 108 (FIG. 4).

Referring back to FIG. 4, because portable device 108 includes RFtransceiver 408, a device capacity may be increased. If a portabledevice employs only a transmitter, it typically repeats its transmissionseveral times, in order to increase the probability that the signal willbe received by RF base station 110. One reason that transmissions do notget received may be collisions with other transmissions by otherportable devices. RFID systems do not typically use time divisionmultiple access (TDMA) as it limits the number of portable devices thesystem can handle. Accordingly, the transmissions are typicallyperformed in a random fashion. Such systems can generally support manytens of thousands of portable devices. 1-D systems (transmitter only)typically transmit each transmission several times to increase theprobability that the signal went through (typically up to 6 times).

In general, RF base station 110 (FIG. 1) may embed commands to portabledevice 108 in an RF transmission. For example, RF base station 110(FIG. 1) may not receive the RF signal due to collisions with otherportable devices 108 that may be transmitting RF signals at the sametime and/or due to signal fading, for example due to enclosure 102 (FIG.1). In an exemplary embodiment, portable device 108 may receive anacknowledgement (ACK) transmission packet from RF base station 110(FIG. 1) that the RF signal transmitted by portable device 108 isreceived and would not need to re-transmit the RF signal. If portabledevice 108 does not include an RF receiver, portable device 108 mayinstead repeatedly transmit the device ID and BS-ID, i.e. the locationinformation, without verification that location information wascorrectly received by RF base station 110 (FIG. 1). In exemplary system100 of the present invention, a 2-D system (that uses a transceiver) isprovided. The combination of a 2D system and synchronization allows theuse of time slots instead of completely random timing, which almosttriples the capacity of the portable device 108.

According to an exemplary embodiment, portable device 108 may include atimer, for example, as part of microcontroller 404. The timer may beused by microcontroller 404 to begin IR detection. Portable device 108may periodically adjust the timer by communicating a request to RF basestation 110 (FIG. 1) a time delay to a next beacon transmission time ora time delay to IR base station 106 (FIG. 1) transmission time. Althougha timer is described with respect to portable device 108, each IR basestation 106 may also include a timer to begin IR transmission.

Portable device 108 may transmit a request to RF base station 110(FIG. 1) via RF transceiver 408 for a time delay to a next beacontransmission time. The RF base station 110 (FIG. 1) may reply toportable device 108 with an RF transmission that includes the time delayto the next beacon transmission time. Portable device 108 may thenactivate RF transceiver 408 to receive the beacon at the received timedelay and may adjust the timer according to the received beacon.

Alternatively, portable device 108 may transmit a request to RF basestation 110 (FIG. 1) via RF transceiver 408 for a time delay to an IRbase station 106 transmission time. The RF base station 110 (FIG. 1) mayreply to portable device 108 with the time delay to the nexttransmission. Portable device 108 may then activate IR detector 402 toreceive the IR transmission from IR base station 106 (FIG. 1) and mayadjust the timer according to the received IR transmission. It iscontemplated that portable device 108 may adjust the timer regularlybased on each received IR transmission or periodically. Although methodsfor timer adjustment are described with respect to portable device 108,similar methods for timer adjustment may also be applied to each IR basestation 106.

Referring now to FIGS. 5, 6A and 6B, a system 500 for locating andidentifying portable devices 108 is shown, according to anotherembodiment of the present invention. FIG. 5 is a block diagram of system500 for locating and identifying portable devices using multiple subnets502; and FIGS. 6A and 6B are graphs of various transmissions insynchronization and communication layers of system 500 as a function oftime.

Referring to FIG. 5, system 500 includes central server 506 that may beconnected to multiple RF base stations 510 using a backbone network, forexample, a LAN. System 500 may include multiple subnets 502, with an RFbase station 510 associated with a respective subnet 502. IR basestations 106 may be associated with one or more subnets 502. Forexample, IR base station 106-A is associated with subnet 502-3 whereasIR base station 106-B is associated with subnets 502-1, 502-2 and 502-3.Portable device 108 on person 504 may stay within one subnet 502 or moveamong subnets 502, for example from subnet 502-2 to subnet 502-4.

Individual clocks on the different nodes (i.e. RF base stations 510, IRbase stations 106 and portable device 108) may be slightly differentfrom each other. As time passes the individual clocks may drift. Asynchronization signal may be used to maintain all of the nodes closeenough to a common time of origin to allow to system to operateefficiently. As described above, an increase in power consumption mayoccur if synchronization is not used. In order to synchronize all nodesto the common time of origin, periodic synchronization signals may betransmitted by the individual RF base stations 510. According to oneembodiment, RF base stations 510 may be synchronized by a wiredconnection, such as a LAN. For example, the system can be synchronizedby a wireless LAN, such as a WiFi network, as a backbone LAN. Althoughin an exemplary embodiment the wireless LAN operates according to theInstitute of Electrical and Electronics Engineers (IEEE) 802.11protocol, it is understood that any suitable wireless LAN may be used.Synchronization by a LAN may not provide enough accuracy because the LANmay have an unknown delay, based on the system load and the technologyused. According to another embodiment, the synchronization signals maybe propagated by dedicated wireless signals sent by RF base stations510.

According to one embodiment of the present invention, the RF basestations 510 may relay the time synchronization to other base stationsthat are scattered around the facility. One of RF base stations 510 maybe designated as a master base station, for example RF base station510-1, and the time of origin is defined through the beacon transmissionof the master base station. All of the remaining RF base stations 510(for example, 510-2, 510,3 and 510-4) that receive the master basestation beacon, transmit respective beacons in predefined time slots,described further below with respect to FIG. 6. For example, each of theremaining RF base stations 510 may transmit a respective beacon inrelatively quick succession.

For example, if 10 RF base stations 510 are provided around a facilityand each beacon duration is about 5 milliseconds with a basic intervalbetween beacons of about 10 milliseconds, the entire period for thebeacons is about 150 milliseconds. A general period of system 500 may bebetween about 3 seconds and 45 seconds. Each beacon may transmit otherinformation, such as an offset time to the time of origin. Accordingly,any portable device 108 or any IR base station 510 that receives thebeacon information may determine how to time the respective transmissionor reception relative to the time of origin, regardless of what beaconthey are tuned to. In this manner, portable device 108 and IR basestations 106 may lock onto different beacons. If portable device 108 orIR base stations 510, in contrast, do not lock onto an absolute time oforigin, portable device 108 or IR base stations 510 may not transmit andreceive information at the same time.

For example, one RF base station (STAR) 510, for example 510-1, may bedesignated as a master and assigned an ID=1. A second STAR 510-2 that isin communication range with the master base station may be assigned anID=2. A third STAR that is in a communication range of ID=1 and/or ID=2may be assigned ID=3.

Master beacon 510-1 (a master transceiver) may originate a beacon thatincludes the originating transceiver ID (for the time of origin). RFbase station 510-2 with ID=2 (a second transceiver) may receive themaster beacon transmission and may originate a second beacon thatincludes the second transceiver ID and the offset from the time themaster beacon was received. RF base station 510-3 with ID=3 (a thirdtransceiver) may receive a beacon transmission from any RF base station510 with a lower ID and may originate a third beacon that includes thethird transceiver and the total time offset from the master beacon.According to another embodiment, the second transceiver with ID=2 mayissue a request for timing synchronization from the master transceiverand the master transceiver may transmit the timing information as areply. Any transceiver with an ID greater than ID=2 may issue a requestfor timing synchronization from another transceiver with an ID less thanthat of the requesting transceiver. The requested RF transceiver maythen reply with the timing synchronization information. It iscontemplated that other suitable methods, for example, methods employedby mesh networks, may also be used to propagate the timing information.One example of mesh networks is described in an article to Cox et al.,entitled “Time Synchronization for ZigBee Networks,” in Proceedings ofthe 37th IEEE Southeastern Symposium on System Theory, Tuskegee, Ala.,March 2005, pp. 135-138, the contents of which are incorporated hereinby reference.

According to another embodiment of the present invention, in order toaccommodate regions that are disconnected (for example, tracking in adifferent building represented by subnet 502-4), a new synchronizingbeacon subnet may be initiated. To initiate subnet 502-4, a new masterbase station beacon generator may be started. This is similar to havinga new system with a new master RF base station to cover a new regionthat is disconnected. This situation may occur when there are twodifferent areas that are physically disconnected, such as two buildings.If there is Ethernet synchronization, then there may be only onenetwork, because all nodes can be synchronized through the Ethernetregardless of the physical distance. Ethernet synchronization, however,is not as accurate and, depending upon the circumstances, may requirelarger batteries to compensate for the extra power consumption. The newsubnet 502-4 may have a new timing, unless there are supplementarysynchronizing signals between the subnets, such as physical wiring orother delay controlled signaling system is available.

According to another embodiment of the present invention, portabledevice 108 may send a request to any RF base station 510 incommunication range for a new beacon location when portable device 108leaves one subnet 502, for example 502-2, and as a result loses theassociated subnet beacon. All RF base stations 510 that receive therequest may communicate the information with server 506. Server 506 thendetermines which RF base station 510 may respond and become the new RFbase station associated with portable device 108. For example, RF basestation 502-4 may be selected. The selected RF base station 510-4 mayrespond to portable device 108 with the associated beacon timinginformation (for example, a length of time from the current response tothe next beacon) or the time to the next IR transmission. It iscontemplated that portable device 106 may continue to assume that it islocked onto a particular RF base station 510 even after losing theassociated beacon. In this manner, an efficient transition to a newsubnet 510 during roaming may be provided. Generally, a clock accuracyfor portable device 108 may be sufficient for a few minutes, to allowthe portable device 108 to acquire a new beacon while still functioningas synchronized IR receiver.

Referring to FIGS. 6A and 6B, transmissions in synchronization andcommunication layers of system 500 are shown as a function of time. Ingeneral, the synchronization layer shown in FIG. 6A may be used forbeacon transmission whereas the communication layer shown in FIG. 6B maybe used for all other needs. In the synchronization layer, transmissionsmay be regularly sent. In the communication layer, transmissions arerandomly accessed. The synchronization and communication layers mayinclude different frequency channels.

In the synchronization layer, synchronization beacons 602 may betransmitted. It is known that two adjacent RF base stations 510typically may not transmit their respective beacons at the same timebecause dead zones may be produced in the area of overlap. Accordingly,beacons 602 may be transmitted in different time slots. For example, 15time slots may be allocated to beacons 602. If there are more RF basestations 510, time slots may be reused. In an exemplary embodiment, a250 ms distance is provided between beacons 602.

In the communication layer, after beacons 602 are transmitted, an IRtime slot 604 for IR transmission from IR base stations 106 is accessed.Portable device 108 to RF base stations 510 communications 606 are thenexecuted. Following time slots 606, an RF base station 510 to server 506communication 608 is then executed.

Referring back to FIG. 5, three different exemplary methods are nextdescribed for synchronization of the RF base stations. In the firstmethod, beacons from RF base stations 510 are periodically transmitted.In the second method, RF base stations 510 may receive timinginformation from other RF base stations 510 by actively requestingsynchronization. RF base stations 510 are typically connected among eachother and to server 506 via an Ethernet backbone network. In the thirdmethod, the timing signal is transmitted on the backbone network.

According to the first exemplary method, the synchronization informationmay propagate through system 500 using beacons. Beacons may be periodictransmissions originating from a master RF base station, for example RFbase station 510-1. The master base station may periodically transmit,for example, every 30 seconds, a transmission that defines the time oforigin. The remaining RF base stations 510 that receive the beacontransmit their own associated beacon (in a different offset time),adding the respective time offset from the time of origin to theassociated beacon transmission. As described above, time slots may bededicated to beacon transmissions and the time slots may be recycled, inorder to limit an amount to of time in the cycle dedicated to beacontransmissions. For example, 15 time slots may be dedicated for beacontransmissions, so that RF base stations 510 which are within a commonreception range may not transmit beacons at the same time. The beaconsmay be transmitted on a dedicated frequency such that there is little orno interference among the beacon transmission and with thecommunications of portable device 108 and IR base stations 106.According to the first method, by using beacons, a channel capacity maybe improved. For example, IR base stations 106 do not have to crowd theair space with requests for timing information. The first method alsoallows broadcasting of information to all portable devices 108simultaneously and does not use a wired network among RF base stations506.

According to the second exemplary method, RF base stations 510 mayreceive timing information from another upstream RF base station 510(i.e. a base station with a lower assigned ID number) by activelyrequesting synchronization information from the upstream RF base station510. According, RF base stations 510 actively request the timinginformation instead of passively listening to periodic beacons. Theselection of an RF base station 510 to provide the timing informationmay be determined as follows. When system 500 is initialized, RF basestations 510 may transmit a broadcast message including a request ofassociation. All upstream RF base stations 510 that receive the requestmay transmit back, in a random time from the time of receiving therequest, a reply to the requesting RF base station 510 that includes theRSSI of the broadcasted request. The requesting RF base station may thendetermine which RF base station 510 it may be associated with based onthe received RSSI values. Alternatively, other methods, such as methodsfor mesh networks may be utilized.

The following section relates to Ethernet synchronization rather thanfor RF synchronization, where the response is always substantiallyimmediate and where, if the signal is not received immediately, thereception window closes. According to an embodiment of the presentinvention, the synchronization may be valid if a time delay between theoriginating request by the RF base station 510 and the received timinginformation is below a threshold. According to the second method, apower consumption by portable device and the IR BS 108 may be higherbecause with the first method the nodes need only listen to beacons andnot have a request/response session. Using the first method (of beacons)also facilitates broadcasting by the RF base stations.

According to the third exemplary method, the synchronization timingsignals among the RF base stations may be transmitted on the backbonenetwork. RF base stations 510 may periodically request a synchronizationtime stamp from server 506 (or from a dedicated time server). In casethe network is temporarily loaded, RF is base stations 510 may repeatthe request a few seconds later to ensure that the round trip time (fromthe request to the receipt of information) is below a system threshold,for example 30 milliseconds, to ensure accurate synchronization.

Synchronization according to the third method does not use a line ofsight between RF base stations 510 and is simple to implement. Thismethod may reduce a number of RF base stations 510 needed but it mayincrease power consumption by portable device 108 because thesynchronization accuracy using a network may be reduced (as compared tothe first and second methods).

Portable device 108 and IR base stations 106 may be associated with oneor more RF base stations 510. According to an exemplary embodiment,portable device 108 (or an IR base station 106) may send a request forassociation to one or more RF base stations 510 that are withintransmission range of portable device 108 (or IR base station 106). AllRF base stations 510 that receive the request may transmit theinformation to central server 506 with the RSSI values of the request.Server 510 may select an RF base station 510 to be associated withportable device 108 (or IR base station 106). The association for the IRbase stations 106 may be determined upon an initialization of system 500because IR base stations 106 are stationary. For portable device 108,however, the association process may be repeated as portable device 108roams among subnets 502 and the associated RF base station 510 changes.

Referring now to FIG. 7, a flow chart shows an exemplary method fordetermining the location and identity of portable device 108, accordingto an embodiment of the present invention. In step 700, portable device108 is initialized with an RF base station 510 (FIG. 5). For example,upon initialization of system 500, portable device 108 may be associatedwith one of the RF base stations 510, as described above. In step 702,portable device 108 is placed in a sleep mode, to conserve powerconsumption by battery 414, for example, by microcontroller 404 (FIG.4). In general, there are two types of sleep modes. In a first type ofsleep mode, portable device 108 goes to sleep if its motion detector 406(FIG. 4) is not active. In a second type of sleep mode, referred to withrespect to FIG. 7, portable device 108 also sleeps between events andwakes up to receive timing synchronization information.

In step 704, portable device 108 wakes up to receive timingsynchronization information, for example, by RF transceiver 408 (FIG. 4)from RF base station 510 (FIG. 5). In step 706, after a time period fromthe received timing synchronization information, one or more IR basestation signals are detected, for example, by IR detector 402 (FIG. 4)controlled by microcontroller 404 (FIG. 4). As described above, eachreceived IR base station signal includes a corresponding BS-ID. In step708, a device ID associated with portable device 108 and the receivedBS-ID are transmitted to the associated RF base station 510 (FIG. 5),for example, by RF transceiver 408 via antenna 412 (FIG. 4).

In step 710, it is determined whether portable device 108 is out of arange of the associated RF base station 510 (FIG. 5), for example, ifportable device 108 has roamed into a different RF base station area ofcoverage 502. If it determined that portable device 108 is not out ofrange of RF base station 510, step 710 proceeds to step 702 and portabledevice 108 is placed in sleep mode.

If it determined that portable device 108 is out of range, step 710proceeds to step 712. In step 712, portable device 108 may be associatedwith another RF base station 510, for example, as described above. Step712 proceeds to step 702 and portable device 108 is placed in sleep modeuntil the next synchronization event occurs.

Referring next to FIGS. 9, 10A, 10B and 10C, an ordering of IR basestations 106 to transmit at different time periods is illustrated,according to another embodiment of the present invention. FIG. 9 is ablock diagram illustrating an ordering of IR base stations 106 in anenclosure such that IR base stations 106 transmit at different timeperiods; and FIGS. 10A, 10B and 10C are diagrams of transmissions versustime of ordered IR base stations 106 and portable device 108.

As shown in FIGS. 8A and 8B and described above, multiple IR basestations 106 transmit the respective BS-ID in IR transmission 804 at thesame time so that portable devices 108 may activate respective IRdetectors 402 (FIG. 4) at the same time as the IR base stationtransmission 804. One problem with such a synchronized approach is thattwo IR base stations 106 may not be provided in a same physical place,because they may interfere with each other. As shown in FIG. 9, IR basestations 106-1, 106-2, 106-3 are each in corridor 104-1. Accordingly, ifthe IR base stations 106 in corridor 104-1 transmit their respective IRtransmissions 804 (FIG. 8A), the IR transmissions 804 may interfere witheach other and may create a dead zone in corridor 104-1. Accordingly, indead zone, portable device 108-1 may not receive either IR transmissionfrom IR base stations 106-1, 106-2, 106-3.

To avoid interference among IR transmissions from IR base stations106-1, 106-2, 106-3 in corridor 104-1, IR base stations 106 may beordered, for example, by even and odd ordered IDs. The odd numbered IRbase stations may transmits in a different delay as compared to the evennumbered IR base stations 106.

Referring to FIGS. 10A and 10B, both even and odd IR base stations 106may receive beacon 1002 and may transmit a communication 1004 with RFbase station 110 (FIG. 9). Even IR base stations 106 may send an IRtransmission 1006 after a period of T/3 from the beacon transmission1002 (FIG. 10A), whereas odd IR base stations 106 may send an IRtransmission 1008 after a second period 2T/3 from the beacontransmission 1002 (FIG. 10B). Although periods T/3 and 2T/3 areillustrated, it is understood that these values are exemplary and thatany suitable period of time between even and odd ordered IR base stationtransmissions 1006, 1008 may be used. The advantage of the variabledelay between or among IR base stations 106 is in the ability for IRbase stations 106 to coexist and function in the same physical openspace. This may improve coverage and allows, for example, contiguouscoverage of open spaces such as corridors 104-1 (FIG. 9).

Referring to FIG. 10C, portable device 108 receives the beacontransmission 1002. After period T/3, IR detector 402 (FIG. 4) may becontrolled to begin receiving an IR signal 1010 from an even IR basestation 106 including the BS-ID. Microcontroller 404 may then control RFtransceiver 408 (FIG. 4) to transmit an RF signal 1012 including theassociated device ID and the received BS-ID. After period 2T/3, IRdetector 402 (FIG. 4) may be controlled to begin receiving an IR signal1014 from an odd IR base station 106 including the BS-ID.Microcontroller 404 may then control RF transceiver 408 (FIG. 4) totransmit an RF signal 1016 including the associated device ID and thereceived BS-ID. After a period T, another beacon 802 may again bereceived by portable device 108 (FIG. 9).

It is understood that an allocation of delays may be provided in othersuitable ways besides even and odd ordered base stations 110, forexample, by assigning a different delay to each IR base station 106based on its identifying number. It is understood that there may be atradeoff between a number of delays versus a power consumption byportable device 108. According to another embodiment, IR base stations106 may be assigned different IR wavelengths corresponding to the oddand even IR base stations 106. Portable device 108 may include multipleIR detectors for detecting the different IR wavelengths and may selectan IR base station 106 having a strongest detected IR signal.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed is:
 1. A system for determining a location and anidentity of a portable device, the system comprising: means fortransmitting timing synchronization information including a plurality ofRF transceivers coupled to a backbone network and a time servergenerating the timing synchronization information; wherein each of theplurality of RF transceivers periodically transmits a request to thetime server to receive the timing synchronization information; aplurality of stationary ultrasonic base stations, each ultrasonic basestation configured to receive the timing synchronization information andto transmit a corresponding ultrasonic location code in a time periodbased on the received timing synchronization information, eachultrasonic location code representative of a location of the respectiveultrasonic base station; and a plurality of portable devices, eachportable device configured to 1) receive the timing synchronizationinformation, 2) detect the ultrasonic location codes from the ultrasonicbase stations and 3) transmit an output signal including a portabledevice ID representative of the portable device and the detectedlocation code, wherein each portable device is synchronized to detectthe ultrasonic location code in the time period based on the receivedtiming synchronization information.
 2. The system according to claim 1,wherein the means for transmitting includes means for receiving theoutput signal from each of the portable devices.
 3. A system fordetermining a location and an identity of a portable device, the systemcomprising: means for transmitting timing synchronization information; aplurality of stationary ultrasonic base stations, each ultrasonic basestation configured to receive the timing synchronization information andto transmit a corresponding ultrasonic location code in a time periodbased on the received timing synchronization information, eachultrasonic location code representative of a location of the respectiveultrasonic base station; and a plurality of portable devices, eachportable device configured to 1) receive the timing synchronizationinformation, 2) detect the ultrasonic location codes from the ultrasonicbase stations and 3) transmit an output signal including a portabledevice ID representative of the portable device and the detectedlocation code, wherein each portable device is synchronized to detectthe ultrasonic location code in the time period based on the receivedtiming synchronization information, and wherein the means fortransmitting includes at least one RF transceiver configured toperiodically transmit beacons in predetermined time slots to theplurality of ultrasonic base stations and the plurality of portabledevices, the beacons including the timing synchronization informationand respective time delay information relative to a unified time oforigin.
 4. A system for determining a location and an identity of aportable device, the system comprising: means for transmitting timingsynchronization information; a plurality of stationary ultrasonic basestations, each ultrasonic base station configured to receive the timingsynchronization information and to transmit a corresponding ultrasoniclocation code in a time period based on the received timingsynchronization information, each ultrasonic location coderepresentative of a location of the respective ultrasonic base station;and a plurality of portable devices, each portable device configuredto 1) receive the timing synchronization information, 2) detect theultrasonic location codes from the ultrasonic base stations and 3)transmit an output signal including a portable device ID representativeof the portable device and the detected location code, wherein the meansfor transmitting includes a plurality of RF transceivers, at least oneof the RF transceivers configured to periodically transmit a request toan other one of the RF transceivers to receive the timingsynchronization information, the other one of the RF transceiversstoring the timing synchronization information; and wherein eachportable device is synchronized to detect the ultrasonic location codein the time period based on the received timing synchronizationinformation.
 5. The system according to claim 1, wherein each ultrasonicbase station includes: an ultrasonic transmitter configured to transmitthe corresponding ultrasonic location code; and a radio frequency (RF)receiver configured to receive the timing synchronization informationfrom the means for transmitting.
 6. The system according to claim 1,wherein each portable device includes: an ultrasonic detector configuredto detect the ultrasonic location codes; and a radio frequency (RF)transceiver configured to 1) transmit the output signal including theportable device ID and the detected ultrasonic location code and 2)receive the timing synchronization information from the means fortransmitting.
 7. A system for determining a location and an identity ofa portable device, the system comprising: means for transmitting timingsynchronization information; a plurality of stationary ultrasonic basestations, each ultrasonic base station configured to receive the timingsynchronization information and to transmit an ultrasonic signalincluding a corresponding ultrasonic location code in a time periodbased on the received timing synchronization information, eachultrasonic location code representative of a location of the respectiveultrasonic base station; and a plurality of portable devices, eachportable device including a corresponding ultrasonic detector and beingconfigured to 1) receive the timing synchronization information, 2)detect the ultrasonic location codes from the ultrasonic signalstransmitted by the ultrasonic base stations and 3) transmit an outputsignal including a portable device ID representative of the portabledevice and the detected location code, wherein each of the portabledevices activates the corresponding ultrasonic detector concurrentlywith a time that each of the plurality of ultrasonic base stationstransmits the corresponding ultrasonic location code of the ultrasonicbase station, based on the received timing synchronization information.8. A system for determining a location and an identity of a portabledevice, the system comprising: means for transmitting timingsynchronization information; a plurality of stationary ultrasonic basestations, each ultrasonic base station configured to receive the timingsynchronization information and to transmit a corresponding ultrasoniclocation code in a time period based on the received timingsynchronization information, each ultrasonic location coderepresentative of a location of the respective ultrasonic base station;and a plurality of portable devices, each portable device configuredto 1) receive the timing synchronization information, 2) detect theultrasonic location codes from the ultrasonic base stations and 3)transmit an output signal including a portable device ID representativeof the portable device and the detected location code, wherein eachportable device is synchronized to detect the ultrasonic location codein the time period based on the received timing synchronizationinformation, and wherein multiple ultrasonic base stations of theplurality of stationary ultrasonic base stations concurrently transmitthe respective ultrasonic location codes of the multiple ultrasonic basestations.
 9. The system according to claim 1, wherein the means fortransmitting include a plurality of RF transceivers that aresynchronized in time through a wired connection as at least a portion ofthe backbone network.
 10. The system according to claim 9, wherein thewired connection is a local area network (LAN).
 11. The system accordingto claim 9, the system further comprising a system server that includesthe time server, wherein the time server provides the timesynchronization information to the RF transceivers upon a request by atleast one of the plurality of RF transceivers.
 12. A system fordetermining a location and an identity of a portable device, the systemcomprising: means for transmitting timing synchronization information; aplurality of stationary ultrasonic base stations, each ultrasonic basestation configured to receive the timing synchronization information andto transmit a corresponding ultrasonic location code in a time periodbased on the received timing synchronization information, eachultrasonic location code representative of a location of the respectiveultrasonic base station, wherein the plurality of ultrasonic basestations transmit the respective ultrasonic location codes periodicallyin a predetermined order, and different ultrasonic base stations amongthe plurality of ultrasonic base stations are activated at timescorresponding to respectively different portions of the orderedtransmission; and a plurality of portable devices, each portable deviceconfigured to 1) receive the timing synchronization information, 2)detect the ultrasonic location codes from the ultrasonic base stationsand 3) transmit an output signal including a portable device IDrepresentative of the portable device and the detected location code,wherein each portable device is synchronized to detect the ultrasoniclocation code in the time period based on the received timingsynchronization information.
 13. A system for determining a location andan identity of a portable device, the system comprising: means fortransmitting timing synchronization information; a plurality ofstationary ultrasonic base stations, each ultrasonic base stationconfigured to receive the timing synchronization information and totransmit a corresponding ultrasonic location code in a time period basedon the received timing synchronization information, each ultrasoniclocation code representative of a location of the respective ultrasonicbase station; and a plurality of portable devices, each portable deviceconfigured to 1) receive the timing synchronization information, 2)detect the ultrasonic location codes from the ultrasonic base stationsand 3) transmit an output signal including a portable device IDrepresentative of the portable device and the detected location code,wherein the timing synchronization information is transmittedperiodically in a cycle and the plurality of ultrasonic base stationsare activated multiple times in each cycle, and wherein each portabledevice is synchronized to detect the ultrasonic location code in thetime period based on the received timing synchronization information.14. The system according to claim 1, wherein at least the plurality ofportable devices or the plurality of ultrasonic base stations areconfigured to be in a sleep mode when not active.
 15. A system fordetermining a location and an identity of a portable device, the systemcomprising: means for transmitting timing synchronization information; aplurality of stationary ultrasonic base stations, each ultrasonic basestation configured to receive the timing synchronization information andto transmit a corresponding ultrasonic location code in a time periodbased on the received timing synchronization information, eachultrasonic location code representative of a location of the respectiveultrasonic base station; and a plurality of portable devices, eachportable device configured to 1) receive the timing synchronizationinformation, 2) detect the ultrasonic location codes from the ultrasonicbase stations and 3) transmit an output signal including a portabledevice ID representative of the portable device and the detectedlocation code, wherein each portable device is synchronized to detectthe ultrasonic location code in the time period based on the receivedtiming synchronization information, and wherein the timingsynchronization information is respectively transmitted to each of theplurality of portable devices and the plurality of ultrasonic basestations in a same transmission packet as an acknowledgement (ACK)indicator responsive to the corresponding output signal that istransmitted by the plurality of portable devices.
 16. An ultrasonic basestation, comprising: a radio frequency (RF) receiver, coupled to a timeserver via a backbone network and configured to receive timingsynchronization information from the time server via the backbonenetwork; and an ultrasonic transmitter configured to transmit anultrasonic location code in a time period based on the received timingsynchronization information, the ultrasonic location code representativeof a location of the ultrasonic base station, wherein the ultrasonicbase station periodically transmits a request to the time server, viathe backbone network, to receive the timing synchronization information.17. The ultrasonic base station according to claim 16, wherein theultrasonic base station is powered by a battery or an external powersupply.
 18. The system according to claim 1, wherein each of theplurality of portable devices includes an ultrasonic receiver configuredto receive ultrasonic signals containing the ultrasonic location codesand is configured to derive the timing synchronization information fromthe received ultrasonic signals.
 19. The system according to claim 3,wherein each of the plurality of portable devices includes an ultrasonicreceiver configured to receive ultrasonic signals containing theultrasonic location codes and is configured to derive the timingsynchronization information from the received ultrasonic signals. 20.The system according to claim 4, wherein each of the plurality ofportable devices includes an ultrasonic receiver configured to receiveultrasonic signals containing the ultrasonic location codes and isconfigured to derive the timing synchronization information from thereceived ultrasonic signals.
 21. The system according to claim 7,wherein the ultrasonic detector of each of the plurality of portabledevices is configured to derive the timing synchronization informationfrom the detected ultrasonic signals.
 22. The system according to claim8, wherein each of the plurality of portable devices includes anultrasonic receiver configured to receive ultrasonic signals containingthe ultrasonic location codes and is configured to derive the timingsynchronization information from the received ultrasonic signals. 23.The system according to claim 12, wherein each of the plurality ofportable devices includes an ultrasonic receiver configured to receiveultrasonic signals containing the ultrasonic location codes and isconfigured to derive the timing synchronization information from thereceived ultrasonic signals.
 24. The system according to claim 13,wherein each of the plurality of portable devices includes an ultrasonicreceiver configured to receive ultrasonic signals containing theultrasonic location codes and is configured to derive the timingsynchronization information from the received ultrasonic signals. 25.The system according to claim 15, wherein each of the plurality ofportable devices includes an ultrasonic receiver configured to receiveultrasonic signals containing the ultrasonic location codes and isconfigured to derive the timing synchronization information from thereceived ultrasonic signals.
 26. An ultrasonic base station, comprising:a radio frequency (RF) receiver configured to receive timingsynchronization information; and an ultrasonic transmitter configured totransmit an ultrasonic location code in a time period based on thereceived timing synchronization information, the ultrasonic locationcode representative of a location of the ultrasonic base station,wherein the RF receiver is configured to receive beacons inpredetermined time slots, the beacons including the timingsynchronization information and respective time delay informationrelative to a unified time of origin.
 27. An ultrasonic base station,comprising: a radio frequency (RF) receiver configured to receive timingsynchronization information; and an ultrasonic transmitter configured totransmit an ultrasonic location code in a time period based on thereceived timing synchronization information, the ultrasonic locationcode representative of a location of the ultrasonic base station,wherein the ultrasonic transmitter of the base station is configured totransmit the ultrasonic location code concurrently with transmission ofother ultrasonic location codes by other ultrasonic transmitters ofrespective other base stations.
 28. An ultrasonic base station,comprising: a radio frequency (RF) receiver configured to receive timingsynchronization information; and an ultrasonic transmitter configured totransmit an ultrasonic location code in a time period based on thereceived timing synchronization information, the ultrasonic locationcode representative of a location of the ultrasonic base station,wherein the RF receiver is configured to receive the timingsynchronization information in a same transmission packet as anacknowledgement (ACK) indicator.